it's necessary to know different (customs) of sending gifts in different countries

问题描述:

it's necessary to know different (customs) of sending gifts in different countries
custom在这句话中的意思是什么,为什么加s

custom这里是指习俗、习惯的意思.“我们有必要了解不同国家送礼物的习俗”
因为涉及到不同国家的不同习俗,所以用复数.
有问题请追加~